A series of structural studies of lipid bilayers with and without incorporated small molecules is proposed. The lipid most commonly found in biological membranes, and in black lipid membranes (BLMs) used for electrical transport studies will be analyzed by X-ray diffraction techniques. The lipids will be studied both partially and fully hydrated in both the gel and liquid crystalline states. Wide-angle diffraction and electron density distributions across the bilayer will be used to obtain the bilayer width, the lipid head group conformation, the hydrocarbon thickness, and the hydrocarbon chain tilt. These parameters will be compared for pure lipid bilayers and bilayers containing specific, well defined small molecules. The effects of various types of small molecules on bilayer organization will be systematically evaluated. Each of the small molecules chosen has important physiological effects on bilayers or membranes. Examples of the classes of small molecules are: 1) non-polar molecules, such as the n-alkanes, which are often incorporated in BLMs for electrical studies, and the aromatic hydrocarbons, many of which are carcinogenic and must cross the plasma membrane to enter the cell; 2) polyvalent cations, which may produce conformational changes in the lipids; 3) lipophilic ions, such as tetraphenylboride which is often used in ion transport studies; and 4) amphipathic molecules, such as cholesterol and the local anesthetics benzyl alcohol and chlorpromazine.